Game concept
Alliance
The game also links it as Alliances, Allies, Allied, Ally.
From the game's encyclopedia
An Alliance is a diplomatic relation between two Rulers. Independent Rulers can call on Allies to join each others Wars.
Alliances are created and maintained through a Marriage or Betrothal, as long as the two characters that are wed are closely enough related to the two Rulers. For example, if a King marries off his daughter to another King's son, the two Kings become Allies.
Lieges can also create Alliances with their Vassals. The Liege cannot call the Vassal to War, but it prevents them from joining Factions.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Alliance is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Alliance is a diplomatic relation between two Rulers." The game also links it as Alliances, Allies, Allied and Ally. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.